1. Field of the Invention
This invention generally relates to color television receivers and, more particularly, to a color temperature control or correction for a color television receiver.
2. Description of the Prior Art
In an NTSC (national television standard committee) system, chromaticity close to the blackbody radiation at a color temperature of 6740.degree. K. is selected to be a standard white color at the image transmitter side. Accordingly, the reference white color upon adjusting the white balance of a color picture tube has to be selected at this color temperature fundamentally.
However, in practice, the white color on the picture screen of the color cathode ray tube which is adjusted as described above is perceived as remarkably "reddish white" under, for example, the illumination of a fluorescent lamp having high color temperature. The reason for this is that human eyes follow illumination light and recognize "bluish white" having high color temperature around the color cathode ray tube as "pure white".
As described above, goodness or badness of the white on the picture screen of the color cathode ray tube greatly depends on the observation condition and also on the preference of each viewer. Generally, it is said that white color having color temperature higher than that of the illumination light is preferred.
While, if the color temperature is set high regardless of the content of a picture image, there arises a problem that a chromatic color, particularly flesh color, becomes bluish and hence unnatural color.
Therefore, in the art it has been proposed to control or correct the color temperature in response to the brightness of a color video signal.
FIG. 1 is a block diagram showing an example of the prior art. In FIG. 1, a composite color video signal is applied to an input terminal 1 and then fed to a Y/C separating circuit 2 where it is separated to a luminance signal Y and a chrominance signal C. The chrominance signal C is color-demodulated by a color demodulating circuit 3 which then produces red, green and blue color difference signals R-Y, G-Y and B-Y, respectively. These color difference signals R-Y, G-Y and B-Y are supplied to a matrix circuit 4. The matrix circuit 4 is also supplied with the luminance signal Y from the Y/C separating circuit 2 through a contrast adjusting variable resistor 5. Thus, the matrix circuit 4 produces red, green and blue primary color signals R, G and B. These red, green and blue primary color signals R, G and B are, respectively, supplied through gain control circuits 6R, 6G and 6B to the cathodes of a cathode ray tube 7. The input vs. output characteristics of the gain control circuits 6R, 6G and 6B are respectively shown in FIGS. 2A, 2B and 2C.
That is, the red primary color signal R is amplified linearly relative to the input. However, although the green primary color signal G and the blue primary color signal B are amplified linearly when the inputs are lower than a predetermined level, when the inputs are higher than the predetermined level, the green primary color signal G and the blue primary color signal B are respectively amplified so as to emphasize green and blue colors. In this case, the blue primary color signal B is emphasized much, while the green primary color signal G may be emphasized a little.
In this case, the above predetermined level is selected to correspond to a portion of high brightness, for example, the telop portion of a white character on a picture screen. The telop portion refers to a character display superimposed upon the television picture on the screen, for example, to display advertising information, announcements, stock market prices and the like.
Thus the telop portion of the white character and the like are made bluish, which is equivalent to the case that the color temperature is raised, white color being reproduced without being reddish. In addition, in the portion of the brightness less than the predetermined level, under the standard color temperature, for example, 9300.degree. K., each primary color signal is supplied to the cathode ray tube so that a flesh color or the like can be prevented from being made bluish.
However, according to such prior art color temperature control or correction system, only the portion of high brightness level such as a portion of a white character and the like can be controlled or corrected and a white color having low brightness level can not be corrected. The reason for this is that this prior art system carries out the color temperature control or correction not for the color saturation level but for the brightness.
Further, since the green and blue primary color signals are passed through the gain control circuits 6G and 6B which are non-linear circuits, there is then a defect that a non-linear characteristic causes color shading.
In addition, there is a further defect that the color temperature is changed by the adjustment of contrast.